Waveguide device

ABSTRACT

A waveguide device includes an indium phosphide substrate, an active layer formed on the indium phosphide substrate, and a cladding layer formed on the active layer, the cladding layer having a ridge structure the side wall of which is configured into a reversed mesa form.

BACKGROUND OF THE INVENTION

The present invention relates to an optical semiconductor device, andmore particularly to an optical semiconductor device suitable for use inan optical communication module, optical communication system or opticalnetwork.

One crystal growth step suffices for the fabrication of aridge-waveguide device. Therefore, the fabrication process of theridge-waveguide device is very simple as compared with that of aburied-hetero-structure device. Hitherto, a satisfactory devicereliability has been reported about an indium phosphide-based ridgewaveguide lasers. However, the conventional indium phosphide ridgelasers using a ridge (denoted by reference numeral 70 in FIG. 9) with arectangular cross section formed by use of a wet etching technique withhydrochloric acid involves the following problems.

(1) Since the width of an electrode contact on an active layer and thewidth of a light emitting layer of the active layer are substantiallythe same, it is required from the aspect of reduction in resistance ofthe device that the width of the cross section of the ridge providingthe width of a light emitting region should be set to a value equal toor larger than 2 μm. Therefore, it is difficult to realize thestabilization of a transverse mode and the reduction of the thresholdcurrent to a value not larger than 10 mA. Further, since the deviceresistance is relatively large, a high-output operation is limited dueto a thermal saturation phenomenon.

(2) Since it is difficult to make the width of the light emitting regionnarrow, it is difficult to reduce a parasitic capacitance of the device.

(3) A lithographic alignment precision required in providing aninsulating layer window for an electrode contact on the ridge is verysevere.

Techniques relevant to the ridge waveguide lasers have been disclosed byInstitute of Electronics, Information and Communication Engineers ofJapan, '93 Spring Conference C-159, March 1993.

SUMMARY OF THE INVENTION

An object of the present invention is provide the device structure of anindium phosphide-based ridge waveguide laser which is realizable by avery simple fabrication method and is capable of a high-output andhigh-speed operation with a low threshold current and to provide amethod of fabricating such a device structure. Another object of thepresent invention is to provide a device structure suitable forapplication to an indium phosphide laser, optical amplifier, opticalmodulator, optical switch, optical detector or integrated waveguidedevice in which at least two of the mentioned devices are integrated andto provide a method of fabricating such a device structure.

To that end, the present inventors have proposed a waveguide structurein which a greatly improved device characteristic is obtained byconfiguring the side wall of an indium phosphide ridge waveguide into areversed mesa form to make the width of an electrode contact large andhence a light emitting region narrow and have proposed a method offabricating such a waveguide structure.

Explanation will now be made of such a waveguide structure and thefabrication method thereof.

As shown in FIG. 1A, an active layer 2 of InGaAsP having a thickness of0.1 μm (and a composition wavelength of 1.30 μm, a spacer layer 3 ofp-type InP having a thickness of 0.1 μm, an etching stopper layer 4 ofInGaAsP having a thickness of 10 nm thickness (and a compositionwavelength of 1.10 μm), a cladding layer 5 of p-type InP having athickness of 2.0 μm and a cap layer 6 of p-type InGaAs having athickness of 0.2 μm are successively formed on a semiconductor substrate1 of n-type (100) InP by use of known techniques.

Next, the cap layer 6 is worked into a stripe structure of 4.4 μm widthby use of a known technique. The direction of the stripe is [011].Subsequently, wet etching with a hydro-bromic acid solution or a mixturesolution of hydro-bromic acid and phosphoric acid is conducted to form aridge waveguide which has a reversed mesa form as showing in FIG. 1B.Thereby, a (111) A surface having the latest speed of etching by theabove etching solution is naturally formed at the side wall of theridge. As a result, the width of a constriction of the mesa providingthe width of a light emitting region can be made narrow or reduced to1.5 μm with the width of an electrode contact being kept wide or 4.4 μm.

Next, a silicon oxide film 7 of 0.5 μm thickness as shown in FIG. 1C isformed on the entire surface of the substrate by use of a knowntechnique. Thereafter, an window 8 of 3.4 μm width passing through thesilicon oxide film is formed on the upper surface of the ridge by use ofan ordinary lithographic and etching process. In this case, since thewidth of the upper surface of the ridge is sufficiently large or 4.4 μm,the lithographic alignment precision is greatly moderated to about 0.5μm as compared with that required in the window forming step of theconventional structure shown in FIG. 9. Also, even in the case where aso-called photoresist etch-back method is used in the window formingstep, the silicon oxide film on the side wall of the ridge of thereversed mesa structure in the present invention is not exposed even inthe case where the etch-back amount becomes large. Therefore, thereliability of the window forming step is greatly improved.

After an electrode forming step, a device having a resonator or cavitylength of 300 μm is cut through a cleavage process. A high-reflectionfilm having a reflectivity of 70% is applied on one end face of thedevice. FIG. 1D shows the form of the cross section of the completeddevice. The fabricated device exhibits a satisfactory characteristicincluding a threshold current of 8 to 10 mA and a slope efficiency of0.40 W/A under a room temperature and continuous operating condition.Also, the forward resistance of the device is about 2 ohms or can bereduced to about one half to two thirds of that of the conventionaldevice shown in FIG. 9. The frequency band of the fabricated device isequal to or larger than 20 GHz as the result of reflection of thereduction of the device resistance and the narrowing of the lightemitting region width to 1.5 μm. A device of 900 μm length includinggrown layers on the lower side thereof attains a high-output operationof 200 mW at the maximum.

As mentioned above, with the configuration of the side wall of the ridgeof the ridge waveguide laser into the reversed mesa form, not only thereduction in threshold current, the increase in efficiency and theincrease in output can easily be realized but also the high-speedfrequency band width of the device can be expanded.

It is needless to say that a similar effect is also obtained in the casewhere the above-mentioned principle of the present invention is appliedto an optical amplifier, an optical modulator, an optical switch, anoptical detector or an integrated waveguide device in which at least twoof the mentioned devices are integrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are perspective views showing an embodiment of awaveguide device of the present invention;

FIGS. 2A to 2D are perspective views showing the structure of oneembodiment of the waveguide device of the present invention;

FIGS. 3A to 3D are perspective views showing the structure of anotherembodiment of the waveguide device of the present invention;

FIG. 4 is a perspective view showing still another embodiment of thewaveguide device of the present invention;

FIG. 5 is a perspective view showing a further embodiment of thewaveguide device of the present invention;

FIG. 6 is a perspective view showing a still further embodiment of thewaveguide device of the present invention;

FIG. 7 is a perspective view showing a furthermore embodiment of thewaveguide device of the present invention;

FIG. 8A is a perspective view showing a moreover embodiment of thewaveguide device of the present invention;

FIG. 8B is a graph showing the characteristic of the waveguide deviceshown in FIG. 8A; and

FIG. 9 is a view for explaining the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described by use of theaccompanying drawings.

(Embodiment 1)

As shown in FIG. 2A, a lower light guiding layer 13 of InGaAsP having athickness of 0.15 μm (and a composition wavelength of 1.10 μm), a7-period multiple quantum well (MQW) structure composed of a well layer14 of InGaAsP having a thickness of 6.0 nm (and a composition wavelengthof 1.37 μm) and a barrier layer 15 of InGaAsP having a thickness of 8 nm(and a composition wavelength of 1.10 μm), an upper light guiding layer16 of InGaAsP having a thickness of 0.05 μm (and a compositionwavelength of 1.10 μm), a cladding layer 17 of p-type InP having athickness of 2.0 μm and a cap layer 18 of p-type InGaAs having athickness of 0.2 μm are successively formed, through known techniques,on a semiconductor substrate 12 of n-type (100) InP having a diffractiongrating 11 thereon.

Next, the cap layer 18 is worked into a stripe structure of 4.4 μm widthby use of a known technique. The direction of the stride is [011].Subsequently, wet etching with a mixture solution of hydro-bromic acidand phosphoric acid is conducted to form a ridge waveguide, as shown inFIG. 2B, which has a (111) A surface as the side wall of the ridge andhas a reversed mesa form in cross section.

Next, a silicon oxide film 19 of 0.15 μm thickness as shown in FIG. 2Cis formed on the entire surface of the substrate by use of a knowntechnique. Thereafter, an window 20 passing through the silicon oxidefilm is formed on the upper surface of the ridge by use of an etch-backmethod. After an electrode forming step, a device having a cavity lengthof 300 μm is cut through a cleavage process. A low-reflection filmhaving a reflectivity of about 1% and a high-reflection film having areflectivity of 90% are respectively formed on front and rear end facesof the device by use of known techniques. FIG. 2D shows the form of thecross section of the completed device.

The fabricated device exhibited a satisfactory oscillationcharacteristic including a threshold current of 6 to 9 mA and a slopeefficiency of 0.45 W/A under a room temperature and continuous operatingcondition. The characteristic under a high temperature condition of 85°C. was also satisfactory or exhibited a threshold current of 20 to 25 mAand a slope efficiency of 0.30 W/A. Also, the evaluation of the deviceas to the longterm reliability demonstrated a stable operation over tenthousand hours under a high temperature condition of 100° C.

(Embodiment 2)

Referring to FIG. 3A, a diffraction grating 31 having a fixed period of240.5 nm is formed on a part of a substrate 32 of n-type (100) InP. AnSiO₂ mask 33 is formed, through a known technique, on a portion of theregion of the substrate 32 where the diffraction grating 31 is formed.The mask 33 includes two stripes each of which has a width of 18 μm andwhich have an interval of 16 μm therebetween. Next, a reduced-pressureorganic metal vapor phase growth method is used to form a lower lightguiding layer 34 of InGaAsP having a thickness of 0.15 μm (and acomposition wavelength of 1.15 μm) and a 7-period MQW structure composedof a well layer 35 of InGaAs having a thickness of 6.5 nm and a latticeconstant shorter than that of InP by 0.3% and a barrier layer 36 ofInGaAsP having a thickness of 8 nm (and a composition wavelength of 1.15μm), as shown in FIG. 3B. After the SiO mask 33 is removed by use of aknown technique, there are grown an upper light guiding layer 37 ofInGaAsP having a thickness of 0.03 μm (and a composition wavelength of1.15 μm), a cladding layer 38 of p-type InP having a thickness of 2.0 μmand a cap layer 39 of p-type InGaAs having a thickness of 0.2 μm.

Next, as shown in FIG. 3C, the cap layer 39 is worked into a stripestructure of 4.4 μm width by use of a known technique, in a mannersimilar to that in the Embodiment 1. The direction of the stripe is[011]. Subsequently, wet etching with a mixture solution of hydro-bromicacid and phosphoric acid is conducted to form a ridge waveguide whichhas a (111) A surface as the side wall of the ridge and has a reversedmesa form in cross section.

Next, a silicon oxide film 40 of 0.6 μm thickness is formed on theentire surface of the substrate by use of a known technique. Thereafter,an window 41 passing through the silicon oxide film is formed on theupper surface of the ridge by use of an etch-back method. After anelectrode forming step, a device having a cavity length of 600 μm is cutthrough a cleavage process. A low-reflection film having a reflectivityof 0.1% and a high-reflection film having a reflectivity of 90% areformed on front and rear end faces of the device by use of knowntechniques, thereby fabricating an optical modulator integrateddistributed feedback laser as shown in FIG. 3D.

The fabricated device exhibited a satisfactory oscillationcharacteristic including a threshold current of 15 to 20 mA and a slopeefficiency of 0.20 W/A under a room temperature and continuous operatingcondition. Also, a modulation band of 20 GHz was obtained as the resultof reflection of the narrowing of the ridge width. Further, as theresult of light transmission at 10 Gb/s using the device of the presentembodiment, there was confirmed a satisfactory transmissioncharacteristic which is free of the deterioration of signal qualityafter transmission.

(Embodiment 3)

Referring to FIG. 4, a lower light guiding layer 52 of n-type InGaAsPhaving a thickness of 0.05 μm (and a composition wavelength of 1.15 μm),a 20-period MQW structure 53 composed of a well layer of InGaAsP havinga thickness of 9 nm (and a composition wavelength of 1.50 μm) and abarrier layer of InP having a thickness of 8 nm, an upper light guidinglayer 54 of InGaAsP having a thickness of 0.05 μm (and a compositionwavelength of 1.15 μm), a cladding layer 55 of p-type InP having athickness of 2.0 μm and a cap layer 56 of p-type InGaAs having athickness of 0.2 μm are formed on an n-type (100) InP substrate 51 byuse of known techniques. Next, as shown in FIG. 4, the cap layer 56 isworked into a branch waveguide structure of 4.0 μm width by use of aknown technique, in a manner similar to that in the Embodiment 1. Thedirection of the waveguide is [011]. Subsequently, wet etching with amixture solution of hydro-bromic acid and phosphoric acid is conductedto form a ridge waveguide which has a (111) A surface as the side wallof the ridge and has a reversed mesa form in cross section.

Next, a silicon oxide film 57 of 0.6 μm thickness is formed on theentire surface of the substrate through a known technique. Thereafter,an window passing through the silicon oxide film is formed on the uppersurface of the ridge by use of an etch-back method. After an electrodeforming step, a device having a cavity length of 1.4 mm is cut through acleavage process. A low-reflection film having a reflectivity of 1% isformed on each of opposite end faces of the device by use of a knowntechnique, thereby fabricating an interference optical modulator.

The fabricated device exhibited a satisfactory modulation characteristichaving an operating voltage of 3 V. The total loss of the device wassmall or 7 dB at the result of reflection of the smooth form of the sidewall of the ridge. Also, a modulation band of 20 GHz was obtained as theresult of reflection of the narrowing of the ridge width. Further, asthe result of light transmission at 10 Gb/s using the device of thepresent embodiment, there was confirmed a satisfactory transmissioncharacteristic which is free of the deterioration of signal qualityafter transmission.

(Embodiment 4)

FIG. 5 shows an embodiment in which a 10-channel laser array isfabricated on the same substrate in a manner substantially similar tothat in the Embodiment 1. The active layer includes a strain MQW(multiple quantum well) structure 61 of InGaAsP with 1.3 μm thicknessformed by use of a known technique. In order to reduce a thresholdcurrent, the width of the light emitting region and the cavity lengthare respectively selected to be 1 μm and 150 μm and high-reflectionfilms having their reflectivities of 80% and 90% are respectively formedon opposite end faces of the device. An oscillation threshold currentand a slope efficiency of the whole channel under a room temperature andcontinuous operating condition were 2 to 3 mA and 0.45 to 0.47 W/A,respectively. Using the fabricated device as a light source for opticalwiring between computer boards, there was confirmed a satisfactorytransmission characteristic in which light emission delay andtransmission delay are reduced.

(Embodiment 5)

FIG. 6 shows an embodiment in which a distributed feedback laser capableof operating at high temperatures higher than 85° C. is fabricated in amanner substantially similar to that in the Embodiment 1. The activelayer includes a strain MQW (multiple quantum well) structure 61 ofInGaAsP with 1.3 μm thickness formed by use of a known technique. Forthe purpose of an increase in output and a satisfactory temperaturecharacteristic, the width of the light emitting region and the cavitylength are respectively selected to be 1.5 μm and 300 μm and low- andhigh-reflection films having their reflectivities of 1% and 90% arerespectively formed on opposite end faces of the device. An oscillationthreshold current and a slope efficiency under a room temperature andcontinuous operating condition were 5 to 8 mA and 0.40 to 0.43 W/A,respectively. Also, an oscillation threshold current and a slopeefficiency under a 100° C. and continuous operating condition were 25 to30 mA and 0.27 to 0.32 W/A, respectively. Using the fabricated device asa light source of a subscriber optical communication system, asatisfactory transmission characteristic was confirmed even at the timeof operation at high temperatures.

(Embodiment 6)

FIG. 7 shows an embodiment in which a high-output laser oscillating at1.48 μm is fabricated in a manner substantially similar to that in theEmbodiment 1. The active layer includes a strain MQW structure 62 ofInGaAsP formed by use of a known technique. For the purpose of anincrease in output and a satisfactory temperature characteristic, thewidth of the light emitting region and the cavity length arerespectively selected to be 1.5 μm and 800 μm and low- andhigh-reflection films having their reflectivities of 5% and 90% arerespectively formed on opposite end faces of the device. An oscillationthreshold current and a slope efficiency under a room temperature andcontinuous operating condition were 25 to 32 mA and 0.40 to 0.43 W/A,respectively. Also, a maximum light output of 400 mW was obtained. Usingthe fabricated device as an excitation light source of an erbium-dopedfiber amplifier, there was confirmed a satisfactory light amplificationcharacteristic which has a low noise intensity.

(Embodiment 7)

FIG. 8A shows the structure of a device in which the high-temperatureoperation characteristic of the distributed feedback laser of theEmbodiment 5 is further improved in a similar manner. The active layerincludes a plurality of well layers 71 of InGaAsP having a compressivestrain of 1%. The thicknesses of the quantum well layers are modulatedor are 6.0, 5.5, 5.0, 4.5 and 4.0 nm at an order from the n-type InPsubstrate 12 side. Each barrier layer has a fixed thickness of 7 nm. Inthis case, a difference in quantum level emission wavelength between themaximum thickness quantum well layer 71a and the minimum thicknessquantum well layer 71e is 30 nm, as apparent referring to from FIG. 8B.The period of a diffraction grating 11 is adjusted so that a differencebetween an oscillating wavelength and a gain peak wavelength is -10 to+10 nm at the room temperature. The multi-layer wafer is worked into aridge-waveguide laser structure similar to that in the Embodiment 5.

In the distributed feedback laser according to the present embodiment, athreshold current and a slope efficiency at an oscillating wavelength of1.3 μm and at the room temperature were 6 to 12 mA and 0.45 to 0.60 W/A,respectively. A difference between the oscillating wavelength and a gainpeak wavelength at the room temperature was in a range between -10 and+10 nm. Also, a device having a threshold current of 15 to 25 mA and aslope efficiency of 0.35 to 0.45 W/A at 85° C. was obtained at a highyield. Stable DFB oscillation was obtained even at -40° C. A side modesuppression ratio was equal to or larger than 35 in the wholetemperature range. This great improvement of the temperaturecharacteristic is caused by the leakage current suppression effectobtained by the reversed-mesa ridge waveguide structure as well as theeffect of expansion of a high-gain wavelength region of the active layerobtained by the modulation of the well layer thickness.

According to a light emitting semiconductor device of the presentinvention, the device characteristic can be greatly improved in such amanner that the side wall of an indium phosphide ridge waveguide havinga low operating current, a low operating voltage and an excellenthigh-speed characteristic is configured into a reversed mesa form tomake the width of an electrode contact large and a light emitting regionnarrow. With the use of the present invention, not only the performanceof the device and the yield are greatly improved but also it is possibleto easily realize the increase in both the capacity and the transmissiondistance of an optical communication system to which the device of thepresent invention is applied.

We claim:
 1. A waveguide device, including:an indium phosphideridge-waveguide formed above a substrate, wherein a side wall of saidridge-waveguide has a reversed mesa form.
 2. A waveguide deviceaccording to claim 1, wherein the ridge is formed using a hydro-bromicacid solution or a mixture solution of hydro-bromic acid and phosphoricacid.
 3. A waveguide device as claimed in claim 1, integrated on asubstrate together with at least one of an indium phosphide laser,optical amplifier, optical modulator, optical switch and opticaldetector.
 4. A waveguide device according to claim 1, wherein the sidewall of said indium phosphide ridge-waveguide protrudes above saidsubstrate without substantially contacting any other structures.
 5. Awaveguide device according to claim 1, wherein the side wail of saidindium phosphide ridge-waveguide is contacted with insulating means. 6.A waveguide device including:a substrate; an indium gallium arsenicphosphide layer formed above said substrate; a ridge shaped indiumphosphide layer formed on said indium gallium arsenic phosphide layer;and an indium gallium arsenic layer formed on an upper surface of saidridge shaped indium phosphide layer, wherein the side wall of said ridgeshaped indium phosphide layer has a reversed mesa form.
 7. A waveguidedevice according to claim 6, further comprising a spacer layer providedbetween said indium gallium arsenic phosphide layer and said indiumphosphide layer.
 8. A waveguide device according to claim 7, furthercomprising an etching stopper layer provided between said indiumphosphide layer and said spacer layer.
 9. A waveguide device accordingto claim 6, wherein said indium phosphide ridge-waveguide protrudesabove said substrate without substantially contacting any otherstructures.
 10. A waveguide device according to claim 6, wherein theside wall of said indium phosphide ridge-waveguide is contacted withinsulating means.
 11. A waveguide device including a ridge-waveguidedevice formed on an indium phosphide substrate, whereina side wall ofsaid ridge includes a reversed mesa form; and wherein the side wall oneach of opposite sides of said ridge includes a crystal surface of a(111) A surface.
 12. A ridge-formed type indium phosphide distributedfeedback semiconductor laser formed on a basis of a waveguide device,whereina side wall of a ridge of the laser includes a reversed mesaform; wherein a light emitting layer of the laser has at least twoquantum well layers; and wherein a quantum level emission wavelength ofat least one of said quantum well layers is different from that of theother quantum well layer.
 13. A waveguide device comprising:an indiumphosphide substrate including a diffraction grating; an active layerformed on said indium phosphide substrate; and a cladding layer formedon said active layer in which said cladding layer includes a ridgestructure, the side wall of which is configured into a reversed mesaform.
 14. A waveguide device according to claim 13, wherein said activelayer has an MQW structure including an alternate superposition of welllayers and barrier layers.
 15. A waveguide device according to claim 14,wherein a thickness of the well layers are successively made thin towarda cladding layer side from a substrate side.